The present invention relates to a planar pn-junction of high electric strength which is located parallel to a surface of a doped semiconductor body of a first conductivity type, and which separates a semiconductor region of a second conductivity type inserted in this semiconductor body from the rest of the semiconductor body. A plurality of field plates is provided in a border area of the semiconductor region which are separated from the surface via an electrically insulating layer and which contact the semiconductor body in the area of a contact hole of this layer.
A planar pn-junction of this type is disclosed in the Jap. Journal of Appl. Physics, Vol. 21 (1982) Suppl. 21.1, p. 97-101. As disclosed, the field plates contact the semiconductor body in close proximity to their inner borders, i.e. the borders facing the semiconductor region. The semiconductor body is additionally provided with a number of guard rings corresponding to the number of field plates, these guard rings being composed of ring shaped semiconductor zones having the same conductivity type as the semiconductor region limited by the pn-junction. The guard rings surround the semiconductor region at respectively increasing distances from the edge of the pn-junction. Each one of the field plates contacts a guard ring allocated to it so that they each determine the potential of the field plate. Between the semiconductor region and the innermost guard ring as well as between the individual guard rings are located subregions or zones of the weakly doped semiconductor body having a conductivity opposite to the semiconductor region, and extending to the surface containing the planar pn-junction. Together with the guard rings, the field plates cause the pn-junction to have a high breakdown voltage in its planar border region upon the application of a blocking voltage. This voltage is fed via two electrodes contacting the semiconductor region and the semiconductor body, respectively. This breakdown voltage approximately corresponds to its volume breakdown voltage which is defined as the voltage level at which the pn-junction breaks down in its part which is parallel to the surface of the semiconductor body, i.e. in the volume of the semiconductor body.
A disadvantage of this prior art pn-structure is that the field plates and the guard rings, which are necessary for achieving a high breakdown voltage, require a relatively large portion of the semiconductor area bordering the edge of the pn-junction.